On 11/15/2010 10:09 AM, Don Jones wrote:
I was wondering about thermal measurements (temperature rise) since everyone
here seems to be fretting about power losses and burning up magnetics,
wirewond inductive elements and band switches.
Thermal measurements are bit tough, but the very practical test of
running power through the device under test and then feeling it for heat
will tell you a lot, and requires no instrumentation beyond your fingers
and brain.
I did this last summer for several multi-turn bi-filar chokes wound on
very lossy cores (#31). The form of the choke is simply a transmission
line wound around a toroid, and I ran 1.5kW through it, inserted between
my Titan 425 and the antenna tuner. With this setup, the choke saw only
the leakage flux that results from the differential signal on the line.
I repeated the test on 160M, 40M, and 10 M. After several minutes of
transmitting, the core showed virtually NO heating at all, and the part
of the wire that was in contact with the core was warm to the touch.
There was NO observable temperature rise on the short lengths of wire
leads that were soldered to coax connectors on each end.
Now, if there was a very high common mode voltage applied to the choke,
there could be considerably more heating, but this heating is I squared
R, so making R large minimizes I, and I squared is changing twice as
fast as R. R is the impedance of the parallel resonance of the choke,
and can be made rather large. That does NOT mean that it can handle
infinite power, but 500W is no problem with very practical, inexpensive,
and excellent chokes that you can build yourself for about $10..
73, Jim
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